等价关系

Multi tool use
Multi tool use









等價關係(equivalence relation)即设R{displaystyle R}R是某個集合A{displaystyle A}A上的一个二元关系。若R{displaystyle R}R满足以下條件:



  1. 自反性:x∈A,  xRx{displaystyle forall xin A,~~xRx}forall xin A,~~xRx

  2. 对称性:x,y∈A,  xRy  ⟹  yRx{displaystyle forall x,yin A,~~xRy~~implies ~~yRx}forall x,yin A,~~xRy~~implies ~~yRx

  3. 传递性:x,y,z∈A,   (xRy  ∧  yRz)  ⟹  xRz{displaystyle forall x,y,zin A,~~~(xRy~~wedge ~~yRz)~~implies ~~xRz}forall x,y,zin A,~~~(xRy~~wedge ~~yRz)~~implies ~~xRz


则称R{displaystyle R}R是一個定义在A{displaystyle A}A上的等价关系。習慣上會把等價關係的符號由R{displaystyle R}R改寫為{displaystyle sim } sim


例如,设A={1,2,…,8}{displaystyle A={1,2,ldots ,8}}A={1,2,ldots ,8},定义A{displaystyle A}A上的关系R{displaystyle R}R如下:


xRy⟺x,y∈A, x≡y(mod3){displaystyle xRyiff forall x,yin A,~xequiv y{pmod {3}}}xRyiff forall x,yin A,~xequiv y{pmod  {3}}

其中x≡y(mod3){displaystyle xequiv y{pmod {3}}}xequiv y{pmod  {3}}叫做x{displaystyle x}xy{displaystyle y}y模3 同餘,即x{displaystyle x}x除以3的餘数与y{displaystyle y}y除以3的餘数相等。例子有1R4, 2R5, 3R6。不难验证R{displaystyle R}RA{displaystyle A}A上的等价关系。


并非所有的二元關係都是等價關係。一個簡單的反例是比較兩個數中哪個較大



  • 沒有自反性:任何一個數不能比自身為較大(n≯n{displaystyle nngtr n}nngtr n

  • 沒有對稱性:如果m>n{displaystyle m>n}m>n,就肯定不能有n>m{displaystyle n>m}n>m




目录






  • 1 不是等价关系的关系的例子


  • 2 参见


  • 3 參考文獻


  • 4 外部連結





不是等价关系的关系的例子


  • 实数之间的"≥"关系满足自反性和传递性,但不满足对称性。例如,7 ≥ 5 无法推出 5 ≥ 7。它是一种全序关系。


参见




  • 当且仅当

  • 等价类

  • 集合划分

  • 商集

  • Apartness relation英语Apartness relation

  • 共轭类

  • Equipollence (geometry)英语Equipollence (geometry)

  • Topological conjugacy英语Topological conjugacy

  • Up to




參考文獻


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  • Brown, Ronald, 2006. Topology and Groupoids. Booksurge LLC. ISBN 1-4196-2722-8.

  • Castellani, E., 2003, "Symmetry and equivalence" in Brading, Katherine, and E. Castellani, eds., Symmetries in Physics: Philosophical Reflections. Cambridge Univ. Press: 422-433.


  • Robert Dilworth and Crawley, Peter, 1973. Algebraic Theory of Lattices. Prentice Hall. Chpt. 12 discusses how equivalence relations arise in lattice theory.

  • Higgins, P.J., 1971. Categories and groupoids. Van Nostrand. Downloadable since 2005 as a TAC Reprint.


  • John Randolph Lucas, 1973. A Treatise on Time and Space. London: Methuen. Section 31.

  • Rosen, Joseph (2008) Symmetry Rules: How Science and Nature are Founded on Symmetry. Springer-Verlag. Mostly chpts. 9,10.


  • Raymond Wilder (1965) Introduction to the Foundations of Mathematics 2nd edition, Chapter 2-8: Axioms defining equivalence, pp 48–50, John Wiley & Sons.




外部連結




  • Hazewinkel, Michiel (编), Equivalence relation, 数学百科全书, Springer, 2001, ISBN 978-1-55608-010-4 

  • Bogomolny, A., "Equivalence Relationship" cut-the-knot. Accessed 1 September 2009


  • Equivalence relation at PlanetMath


  • Binary matrices representing equivalence relations at OEIS.





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